Adds path for Launcher to request Assist override.

Launcher -> SysUI (ISystemUiProxy):
 - setAssistantOverridesRequested(int[] invocationTypes)

SysUI -> Launcher (IOverviewProxy):
 - onAssistantOverrideInvoked(int invocationType)

Override logic is consolidated in AssistManager, which stores
the invocation types that Launcher has requested to override.
Any startAssist calls to those types are sent over OverviewProxy.
Otherwise the existing flow is used.

Bug: 295874732
Test: Manual
Change-Id: I1707cb0e5903bbb9bb3392e2954514e259f0312d